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The Double­edged Ozone and Climate Change 1. Stratospheric cooling ­ its connection with global warming 2. Importance of stratospheric cooling Written by: LEUNG Wing­mo 1.
We love and hate ozone because it plays the dual role of a devil and an angel. We all know that
without the ozone layer high up in the atmosphere (the stratosphere is the scientific term, roughly
from 10 ­ 50 km from the surface), the ultraviolet radiation will be too strong for the survival of many
life species on the earth's surface. So ozone is our guardian angel when it is high aloft. But ozone
at ground level is a notorious pollutant. It can cause a lot of problems with our respiratory symptoms,
including coughing, throat irritation, discomfort in the chest, wheezing, or shortness of breath, and
can produce long term health detriment. But little do people know that ozone plays a role in climate change, and vice versa. First of all, ozone is the third most important greenhouse gas (GHG) in terms of its contribution to
global warming, behind carbon dioxide and methane (Figure 1). So, the more ozone and other
GHG at the lower atmosphere, the warmer it gets. This is obvious enough. Figure 1: The importance of various factors contributing to global warming (or cooling) is expressed
in terms of radiative forcing (RF: +ve value denotes warming effect, ­ve value denotes cooling) estimates. (From Summary for Policymakers, 4th Assessment Report, IPCC)
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2. Stratospheric cooling ­ its connection with global warming When the surface warms up, the opposite occurs at the stratosphere. The lower stratosphere
appears to be cooling by about 0.5oC per decade, and the cooling trend seems to be increasing. One of the reasons for the cooling is the occurrence of the ozone hole in recent decades. The less
the ozone, which generates heat in the stratosphere both by absorbing the sun's ultraviolet radiation
and by absorbing infrared radiation from the lower atmosphere (troposphere), the cooler the
stratosphere becomes. The other reason is, strangely, because of global warming at the lower atmosphere. With the
building up of greenhouse gases in the lower atmosphere in recent decades, the infra­red radiation
from the surface of the Earth is trapped in the lower atmosphere, and little of the heat reached the
stratosphere. As a result, the stratosphere cools. A similar phenomenon occurs at our sister planet,
Venus. Venus's atmosphere is virtually made up of carbon dioxide, the resulting greenhouse effect
is so strong that Venus has a surface temperature of over 400oC. However, Venus's upper
atmosphere is a few times colder than Earth's upper atmosphere. Back to content
3. Importance of stratospheric cooling The largest ozone losses occur in the polar stratosphere in the winter and early spring, when the
temperature at the stratosphere is very low. When temperatures drop below ­78oC, thin clouds ­
polar stratospheric clouds (PSC, Figure 2 and Figure 3) ­ form. These high altitude clouds destroy
ozone in two ways ­ they provide a surface which converts some forms of chlorine into reactive,
ozone­destroying forms, and they remove nitrogen compounds that moderate the destructive impact
of chlorine. This explains the formation of the ozone "hole". In late spring, temperatures begin to
rise, the cloud evaporates, and the ozone layer starts to recover. Since global warming causes stratospheric cooling which favours the formation of PSC, which in
turn leads to more frequent occurrence of ozone holes, our insatiable need for fossil fuels and the
increase in GHG could delay the recovery of the ozone layer. So, what we are facing are not only
the various impacts of extreme weather caused by climate change, but also the detrimental effects
to the health of mankind as a result of depletion of the ozone layer. Figure 2: Polar stratospheric cloud photographed by Lamont Poole, NASA Langley Research Center (Source: NASA)
Figure 3: Polar stratospheric clouds photographed by Mark R. Schoeberl, NASA GSFC (Source: NASA)
Back to content References:
1. 2. Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) (IPCC, 2007).